Impact printer with yielding platen

ABSTRACT

An impact printer (30) comprises a printhead (31) traversing along a guide for printing a line on a document (32) of thickness S, and is provided with contrast means for supporting the document (32) against the head (31) and for contrasting the impact force exerted by the latter against the document. The printhead (31) bears a spacing roller (34) which causes the contrast means to yield in order to compensate for thickness S, which varies according to the document (32), and thus maintain the distance D between the head (31) and the document (32) constant. The contrast means comprise a flexible platen (33) attached yieldingly at the ends, of length at least equal to or greater than that of the print line and having a variable section frame so that the pressure generated between the platen (33) and the roller (34) is substantially constant all along the print line.

FIELD OF THE INVENTION

The present invention relates to an impact printer comprising aprinthead that travels along a guide for printing a line of print ofdetermined length on a document, contrast means for supporting thedocument against the head and contrasting the impulse exerted by thehead during printing of the line, spacing means borne by the printheadand cooperating with the contrast means for maintaining a constantdistance between the head and the document along the line of print, andelastic pressure means for applying pressure between the spacing meansand the contrast means.

RELATED TECHNOLOGICAL ART

A known printer of characteristics similar to those described above isrepresented in diagram form in FIG. 1, wherein it is labelled 10. Aframe 11 of the printer 10 supports two springs 12 that act on the ends18 of a platen 13, ideally rigid type and on which a document 14 rests.A needle printhead 16, of known type, moves in a fixed rectilineartrajectory along the document 14 for printing characters on the document14 through the interposition of an ink ribbon not shown in the drawings,and bears a spacing roller 17, which presses the document 14 against theplaten 13 during movement of the head 16. In this way, the springs 12become deformed and thus allow the platen 13 to move with respect to thehead 16 and adapt itself to thickness S of the document 14, so that theroller 17 keeps the document 14 at a constant distance D from the head16, regardless of thickness S of the document.

A pressure P that the roller 17 exerts on the document 14 variesconsiderably during movement of the head 16. In fact the roller 17, whenin proximity of one of the two ends 18 of the platen 13, is contrastedsolely by a force corresponding to the elastic reaction of one of thesprings 12, whereas, as the head 16 approaches the centre of the platen13, as indicated by the dashed line in FIG. 1, this force tends toincrease and become theoretically, assuming null flexibility of theplaten 13, twice that at the ends of the platen 13 and corresponding tothe yielding of both springs 12.

The pattern of P is illustrated in graphic form in FIG. 2 by a curve 19which, for a given thickness S of the document 14, represents thevariation of the pressure generated between the roller 17 and the platen13, along an entire length L of the platen 13; the value assumed by thepressure P in proximity of the two ends 18 of the platen 13 is indicatedwith P_(est), while the value assumed at the centre L/2 is indicatedwith P_(max).

Variation of the pressure of the spacing roller on the document duringhead movement gives rise to numerous drawbacks, notably:

the likelihood of the roller 17 making a mark on the document,especially if the latter comprises carbon copy sheets or tracing paper,by passing over it during movement of the head or during feeding of thedocument when a line feed is effected;

variable printing density along the print line when the printhead isimpact type, a needle printer for example. In this case, in fact, theelements of the printhead that touch the document exchange energy withthe bar, and printing density may be greater at the centre of the line,where the platen contrasts the head with a pressure twice that exertedon the ends, where printing may appear faint. To the detriment ofprinting density, in fact, the platen tends to dynamically absorb agreater portion of the energy of the head impact elements at its ends,where it contrasts the head with a pressure markedly lower(approximately half) than at the centre of the bar.

Moreover, if a sufficient printing density is also desired in proximityof the ends of the platen and not solely at the centre, the forceexerted by the springs on the ends must be increased, but thisrequirement conflicts with that of reducing the force in order to avoidthe roller causing marks on the document at the centre of the bar.

SUMMARY OF THE INVENTION

One object of the present invention is to eliminate the drawbacksdescribed in the foregoing, and this object is achieved by the printerof the invention, which is characterized by the fact that the pressurebetween the spacing means and the contrast means is substantiallyconstant along the entire length of the print line.

According to a further characteristic of the invention, the contrastmeans comprise a flexible platen having its two ends resting on yieldingsupports and placed apart at a distance at least equal to length of theprint line and, according to yet another characteristic, the value forthe flexible platen of the product of the modulus of elasticity of thematerial from which the platen is manufactured by the moment of inertiaof the section of the platen with respect to its longitudinalbarycentric plane, is variable and tends to increase in the directionfrom the ends of the platen towards the centre.

These and other characteristics of the invention will become apparentfrom the description that follows, provided by way of non-exhaustiveexample, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of an impact printer comprising aplaten manufactured according to the prior art;

FIG. 2 illustrates the pattern of pressure between the head and theplaten in the printer of FIG. 1;

FIG. 3 illustrates in partial manner a perspective view of the printerand of the platen according to the invention;

FIG. 4 illustrates the pattern of pressure between the printhead and theplaten of FIG. 3;

FIG. 5 illustrates a partial longitudinal view of the platen, accordingto a first embodiment of the invention;

FIG. 6 illustrates a transverse section of the platen of FIG. 5;

FIG. 7 illustrates a front view, showing details of some of the membersof FIG. 3;

FIG. 8 illustrates a sectional view of a second embodiment of the platenaccording to the invention; and

FIG. 9 illustrates a perspective view of a third embodiment of theplaten according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 3, a printer 30 is provided with an impact head31, for example but not necessarily, a needle printhead, and with aplaten 33, manufactured from a material having a modulus of elasticityE, on which rests a document 32 to be printed and having a thickness S.The head 31 bears a spacing roller 34 which presses on the document 32to push it against the platen 33 and maintain it at a fixed distance Dfrom the head 31. In turn, the platen 33 is set at its ends on yieldingsupports 36, one only of which is illustrated in FIG. 3, and eachcomprising a spring 37 and an arm 38 oscillating on a fixed frame 39 ofthe printer 30. As may be seen plainly in FIG. 7, the spring 37 is setbetween the arm 38 and the platen 33, whereas the arm 38 has one endattached pivotingly to the platen 33 by means of a pin 43.

In addition, the arm 38 is hinged in an intermediate area between itsends to the frame 39 by means of another pin 44. Thus the arm 38 issuitable for oscillating about frame 39, causing the spring 37 to becomedeformed, and allowing the platen 33 to move with respect to the head34, to compensate for thickness S of the documents 32 placed between theplaten 33 and the head 31. For limiting travel of the platen 33 whenevernot contrasted by roller 34, an abutment element 40 is provided attachedto the platen 33, which extends through a hole 45 in the frame 39 and issuitable for abutting against the latter when the head 31 is notdisposed above the platen 33 and as a result the roller 34 is notpressing against it.

In addition, the platen 33 is provided with two guide areas 41 and 42(FIG. 6), arranged along and on opposite sides of the platen itself,with the purpose of guiding passage of the document 32 under the head 31so that it does not become jammed.

FIGS. 5 and 6 illustrate the platen 33 in longitudinal and transverseview respectively, and show that the platen 33, according to a firstembodiment of the invention, has a T section, height h of which tends toincrease, in the direction from the ends of the platen 33 towards thecentre. A generic section VI--VI of the platen 33, like that illustratedin FIG. 6, defines a horizontal barycentric plane 51 passing through thecentre of gravity of this section.

In this embodiment, as is well known to those skilled in the sector art,the moment of inertia J of the section with respect to the plane 51increases in the direction from the ends of the platen towards thecentre, and accordingly also the product of the moment of inertia J bythe modulus of elasticity E of the material from which the platen ismanufactured increases from the ends towards the centre of the platen.Moreover, as is also known to those skilled in the sector art, theproduct of the modulus of elasticity E by the moment of inertia Jmentioned before is inversely proportional to the capability of theplaten 33 to bend under a determined pressure. In other words, the lowerthe product of E by J, the greater the flexibility of each section ofthe platen 33 or, to put it the other way round, the higher the productof E by J, the greater the rigidity offered.

The frame of the platen 33, according to this first embodiment,determines a pattern of the product of E by J such that differentdegrees of flexibility are given to the said platen, which is thusselectively deformable under the pressure exerted on it by the spacingroller 34, and becomes suitable for cooperating with the yielding means36 so as to contrast the spacing roller 34 with a substantially constantpressure P over its entire length, when the roller 34 moves along theplaten. In other words, the platen 33 bends to a different degree underthe thrust of the roller 34, while during this bending action theyielding supports 36 yield, and overall behaviour of the platen 33 andof the supports 36 results in a substantially constant pattern of thepressure P all along the entire length L of the platen 33.

This substantially constant behaviour of the pressure P, for a giventhickness S of the document 32, is illustrated in graphic form in FIG. 4by the curve 20 which shows how the value of P varies, along the entirelength L of the platen 33, between P_(max) and P_(est) (where P_(max)and P_(est) are respectively the maximum pressure and the pressure atthe ends of the platen 33), such that the following expression holds:

    (P.sub.max /P.sub.est)<2;                                  (1)

If the thickness S increases, the average pressure P also increases, butregardless of the thickness of the document (in practical applications,generally between 0.1 mm and 4 mm), the above expression (1) will alwaysbe verified.

The curve 20 is the result of experimental measurements performed by theinventors on a platen constructed as described previously, measurementsobtained by moving a dynamometer along the platen and recording at eachpoint the pressure needed to cause a determined constant yielding ofthat point. Also illustrated in FIG. 4, by means of the curves 21, aresome patterns of the pressure P for different values of the thickness Sof the document 32.

Verifications were also made of printing quality using a printer inwhich dimensions were selected for the yielding supports 36 and theflexible platen 33 such that a substantially constant force is generatedbetween the platen 33 and the roller 34 along the entire print stroke ofthe roller 34, in the vicinity of a value 3 Nw, over a wide range ofthicknesses S of documents handled, from about one tenth of a millimetreto 4 mm.

These thicknesses are those of documents, ranging from a single sheet toa banking passbook, whereas the indicative value of 3 Nw defined for thecontact force between roller and document was selected both to contrastin optimum fashion the force exerted by the impact members of the head31 on the platen 33 (a force which on average is always less than 2 Nw),avoiding excessive absorption by the platen of the energy of theseimpact members and the resultant reduction in printing density, and alsoto avoid the defect of the roller marking the document.

The platen 33 is preferably manufactured from a plastic material, butmay have a mixed type frame, with metallic inserts distributed orlocated over the entire width of the platen. Also inserted at the toppart of the platen 33 is a strip 52 of special plastic, which generallyhas the function of improving the surface characteristics of the platen33 in the area that the document 32 rests on.

FIG. 8 illustrates a second embodiment of the platen, in which theplaten has an upside-down U section 54, manufactured from a plasticmaterial, and has a longitudinal profiled part 53, made, for example, ofa metal or plastic sheet, arranged inside the U section. The profiledpart 53 and the part 54 of plastic material together define theflexibility characteristics of the platen 33, suitable for determining asubstantially constant pressure pattern.

FIG. 9 represents a third embodiment of the platen, formed by a body 56of yielding material, for example rubber. A profiled strip 57, made, forexample, of a metal or plastic sheet, is attached on the upper part ofthe body 56 and defines a support plane 58 for the document 32 and twoguide planes 59 and 61 for guiding the document 32 towards the plane 58and to the outlet from the plane. The strip 57 is additionally providedwith a plurality of notches 62 in the planes 59 and 61, suitable forconferring greater flexibility to the strip 57 during deformation of theplaten 33.

It will be apparent that changes and/or improvements may be made to theprinter of this invention, without exiting from the scope of theinvention.

We claim:
 1. An impact printer comprising:an impact printhead forprinting a print line of determined length (L) on a document; a flexibleplaten manufactured from a material with modulus of elasticity E, saidplaten having two opposite ends, the distance between said opposite endsbeing at least equal to said length (L) of said print line, and acentre, a transversal section of said flexible platen having a moment ofinertia J with respect to a longitudinal barycentric plane; spacingmeans borne by said printhead and cooperating with said flexible platenfor maintaining a constant distance between said printhead and adocument along said print line; resilient support means for resilientlysupporting said two ends of said flexible platen, and means formaintaining a constant pattern of a pressure (P) between said spacingmeans and said flexible platen along said length (L) of said print line,said means including dimensioning means for said flexible platen suchthat a value for the product of said modulus of elasticity E by saidmoment of inertia J has a maximum at said centre and decreases only inboth directions from said centre, in the direction of said print linetowards said ends of said platen.
 2. An impact printer according toclaim 1, wherein said transversal section of said flexible platen has aU shape.
 3. An impact printer according to claim 1, wherein said platencomprises a part manufactured with a yielding material, and a profiledstrip arranged in a top portion of said platen and cooperating with saidpart for supporting a document against said printhead.
 4. An impactprinter according to claim 1, wherein said resilient support meanscomprise an oscillating arm and a spring, said arm having a first end, asecond end opposite to said first end, and a hinging pin intermediatebetween said first and said second end, said first end being pivotinglyattached to said platen by means of a pin, and said spring being setbetween said platen and said second end.
 5. An impact printer accordingto claim 1, wherein said transversal section of said flexible platen hasa T shape.
 6. An impact printer according to claim 5, wherein a stem ofsaid T shaped transversal section of said flexible platen has a heightof a value decreasing in both directions from said centre towards saidends of said platen.
 7. A platen for supporting a document against aspacing roller rotatively connected to an impact printhead in an impactprinter, said platen having a length, two opposite ends and a centre,along the print line of the document said platen resting at said ends onresilient supports, said platen being manufactured from a material withmodulus of elasticity E, a transversal section of said platen having amoment of inertia J with respect to a longitudinal barycentric plane,wherein said platen is dimensioned such that a value for a product ofsaid modulus of elasticity E by said moment of inertia J has a maximumat said centre and decreases only along the print line in bothdirections from said centre toward said ends of said platen.
 8. A platenaccording to claim 7, wherein said resilient supports comprise anoscillating arm and a spring, said arm having a first end, a second endopposite to said first end, and a hinging pin intermediate between saidfirst and said second end, said first end being pivotingly attached tosaid platen by means of a pin, and said spring being set between saidplaten and said second end.
 9. A platen according to claim 7, whereinsaid transversal section of said flexible platen has a U shape.
 10. Aplaten according to claim 7, wherein said platen comprises a partmanufactured with a yielding material, and a profiled strip arranged ina top portion of said platen and cooperating with said part forsupporting a document against said spacing roller rotatively connectedto said impact printhead.
 11. A platen according to claim 7, whereinsaid transversal section of said flexible platen has a T shape.
 12. Aplaten according to claim 11, wherein a stem of said T shapedtransversal section of said flexible platen has a height of a valuedecreasing in both directions from said centre towards said ends of saidplaten.